Nested scopes and class variables

Nick Coghlan ncoghlan at iinet.net.au
Mon Jan 31 11:15:42 CET 2005


Dave Benjamin wrote:
> I ran into an odd little edge case while experimenting with functions that
> create classes on the fly (don't ask me why):

It gets even kookier:


Py> x = 5
Py> def f(y):
...   class C(object):
...     x = x
...   print C.x
...
Py> f(5) # OK with x bound at global
5

Py> def f(x):
...   class C(object):
...     x = x
...   print C.x
...
Py> f(6) # Oops, ignores the argument!
5

Py> def f(y):
...   class C(object):
...     x = y
...   print C.x
...
Py> f(6) # OK with a different name
6
Py> y = 5
Py> def f(y):
...   class C(object):
...     x = y
...   print C.x
...
Py> f(6) # Correctly use the nearest scope
6

That second case is the disturbing one - the class definition has silently 
picked up the *global* binding of x, whereas the programmer presumably meant the 
function argument.

With a nested function definition (instead of a class definition), notice that 
*both* of the first two cases will generate an UnboundLocalError.

Anyway, the Python builtin disassembler is very handy when looking at behaviour 
like this (I've truncated the dis output below after the interesting section):

Py> import dis
Py> def f1(x):
...   class C(object):
...     x = x
...   print C.x
...
Py> def f2(y):
...   class C(object):
...     x = y
...   print C.x
...

Py> dis.dis(f1)
   2           0 LOAD_CONST               1 ('C')
               3 LOAD_GLOBAL              0 (object)
               6 BUILD_TUPLE              1
               9 LOAD_CONST               2 (<code object C at 00B3E3E0, file "<s
tdin>", line 2>)
[...]

Py> dis.dis(f2)
   2           0 LOAD_CONST               1 ('C')
               3 LOAD_GLOBAL              0 (object)
               6 BUILD_TUPLE              1
               9 LOAD_CLOSURE             0 (y)
              12 LOAD_CONST               2 (<code object C at 00B3E020, file "<s
tdin>", line 2>)
[...]

Notice the extra LOAD_CLOSURE call in the second version of the code. What if we 
define a function instead of a class?:

Py> def f3(x):
...   def f():
...     x = x
...     print x
...   f()
...

Py> def f4(y):
...   def f():
...     x = y
...     print x
...   f()
...

Py> dis.dis(f3)
   2           0 LOAD_CONST               1 (<code object f at 00B3EA60, file "<s
tdin>", line 2>)
[...]

Py> dis.dis(f4)
   2           0 LOAD_CLOSURE             0 (y)
               3 LOAD_CONST               1 (<code object f at 00B3EC60, file "<s
tdin>", line 2>)
[...]

Again, we have the extra load closure call. So why does the function version 
give us an unbound local error, while the class version doesn't?. Again, we look 
at the bytecode - this time of the corresponding internal code objects:

Py> dis.dis(f1.func_code.co_consts[2])
   2           0 LOAD_GLOBAL              0 (__name__)
               3 STORE_NAME               1 (__module__)

   3           6 LOAD_NAME                2 (x)
               9 STORE_NAME               2 (x)
              12 LOAD_LOCALS
              13 RETURN_VALUE

Py> dis.dis(f3.func_code.co_consts[1])
   3           0 LOAD_FAST                0 (x)
               3 STORE_FAST               0 (x)

   4           6 LOAD_FAST                0 (x)
               9 PRINT_ITEM
              10 PRINT_NEWLINE
              11 LOAD_CONST               0 (None)
              14 RETURN_VALUE

In this case, it's the LOAD_FAST opcode that blows up, while the LOAD_NAME falls 
back on the globals and then the builtins. Looking at the class based version 
that works also lets us see why:

Py> dis.dis(f2.func_code.co_consts[2])
   2           0 LOAD_GLOBAL              0 (__name__)
               3 STORE_NAME               1 (__module__)

   3           6 LOAD_DEREF               0 (y)
               9 STORE_NAME               3 (x)
              12 LOAD_LOCALS
              13 RETURN_VALUE

Here we can see the "LOAD_DEREF" instead of the "LOAD_NAME" that was present in 
the version where the same name is reused. The dereference presumably picks up 
the closure noted earlier.

I vote bug. If the assignment is going to be allowed, it should respect the 
nested scopes.

Cheers,
Nick.

-- 
Nick Coghlan   |   ncoghlan at email.com   |   Brisbane, Australia
---------------------------------------------------------------
             http://boredomandlaziness.skystorm.net



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